1) Field of the Invention
The invention relates to a method for controlling a moisture profile of a moving paper web during manufacture.
The invention further relates to an apparatus for controlling the moisture profile of the paper web on a paper machine that comprises at least a headbox, a former, a press unit, a dryer unit and a reel.
2) Description of Related Art
As velocities increase, the quality of paper and runability both on the paper machine and in the further processing become more and more important variables. At the same time the number of online processes grow in paper machine concepts, both in new machines and in renovations of old paper machines. For instance, in online after-treatment, particularly in online multi-nip calendering of demanding grades, extremely high requirements are set for cross machine direction (CD) and machine direction (MD) profiles of the paper. A moisture profile is one of these profiles. A poor moisture profile is most typically the major cause of variation in the physical properties as well as in the tension profile.
Currently, the moisture profile of a paper web is typically measured after the dryer section, and the moisture profile is adjusted, e.g., by a steam box of the press section or later at the end of the dryer section, or after the dryer section. The article xe2x80x9cOptimize or compromise? The art of former section tuningxe2x80x9d, Odell M. 51st Appita Annual General Conference 1997 Proceedings, Volume 1, discloses one solution of this kind. However, this solution is not sufficiently able to eliminate defects in the properties of the finished paper resulting from the combined effect of draw differences and moisture profile defects on the paper machine. The combined effect of the draws in the dryer section and the moisture profile defects, as well as drying shrinkage, pose the greatest problem. The combined effect is due to amounts of various tensile components stored in the paper, which produce defects in the stretch at break profile, tension profile and tensile strength profile of the web, such that the greater the draw, the greater the defects in the finished paper.
The article xe2x80x9cKosteusprofiilin mittaus paperikoneen mxc3xa4rxc3xa4ssxc3xa4 pxc3xa4xc3xa4ssxc3xa4 (Moisture profile measurement at the wet end of the paper machine)xe2x80x9d, Riikka Gerlander, Paperi ja Puuxe2x80x94Paper and Timber, vol. 82, issue June 2000, sets forth moisture profile measurement of a paper web immediately after the press section. A measuring head traversing the paper web in the cross machine direction is employed in the measurement. The publication also refers to a possibility of controlling the operation of the steam box by means of the data collected on the moisture profile. However, this solution is not sufficiently able to eliminate defects in the properties of the finished paper resulting from the combined effect of draw differences and moisture profile defects on the paper machine. In addition, a traversing measuring method is excessively slow for providing an effective and rapid control.
DE publication 29,923,284 discloses a paper machine in which a paper web is conveyed from a former, through a press section, to the beginning of a dryer section such that the paper web is always supported throughout by at least a roll or a band. A baby press comprising a suction roll and a press roll is arranged in connection with the end section of the former. A steam box is arranged between the former and the press section. A sensor, which measures a moisture profile of a paper web, is arranged at the beginning of the dryer section. The measurement can be used for controlling the steam box, the partial vacuum of the suction roll and the pressing force of the press roll. The presented solution is not able to adjust the moisture profile of the paper web effectively and rapidly enough in order to sufficiently eliminate defects in the properties of the finished paper resulting from the combined effect of differences in paper machine draw and moisture profile defects.
FI publication 104,988 discloses a solution, in which a surface temperature of first drying cylinders of a dryer section and dry matter/moisture content and temperature of a paper web are measured prior to first drying cylinders. The paper web moisture is controlled on the basis of these measurements. However, it would be useful to be able to control the paper web moisture profile more rapidly and effectively, so that defects in the finished paper resulting from the combined effect of the differences in paper machine draw and moisture profile defects could be eliminated efficiently enough.
Currently, one more typical approach is to overdry the paper by means of the dryer section of the paper machine so as to equalize the moisture profile of the paper. Overdrying deteriorates the quality of the paper, and furthermore, when the dryer section is used for overdrying the paper, the drying capacity of the dryer section is relatively limited.
U.S. Pat. Nos. 4,801,809 and 5,172,005 disclose solutions, in which paper web properties are measured at several points simultaneously utilizing CCD or other camera technology. DE publication 19,830,323 discloses measuring the thickness of a paper web such that a measuring beam is directed to the paper web with optical fibers and the beam from the web is directed to a measuring sensor with optical fibers. A plurality of optical fibers are thus arranged in parallel extending substantially across the whole paper web in the cross machine direction. Various solutions for measuring paper web properties have also been set forth in FI publication 73,319, DE publication 3,336,659, WO publication 98/40727 and U.S. Pat. No. 5,019,710. However, none of the above-mentioned publications discloses a solution for controlling a moisture profile of a paper web.
The object of the present invention is to provide an improved solution for controlling the moisture profile of the paper web.
The method of the invention is characterized by disposing a measuring device adjacent the paper web, the measuring device comprising optical fibers through each of which radiation in the infrared range is transmitted to the paper web and optical fibers through each of which the radiation is transmitted from the paper web at a measuring point. The measuring device also includes a plurality of parallel measuring heads in which ends of the optical fibers are respectively arranged, the measuring heads being spaced apart in a cross-direction of the paper web. The method comprises steps of measuring the moisture profile of the paper web with the measuring device prior to drying the paper web, wherein the measuring heads are moved back and forth over part of the web width, perpendicularly to the travel direction of the paper web, such that the measuring heads substantially simultaneously measure web moisture at a plurality of measuring points distributed across substantially a full width of the paper web so as to measure a cross-direction profile of the web moisture across substantially the full width of the web; and controlling the moisture profile of the paper web based on the cross-direction profile measured before drying the paper web.
Further, an apparatus in accordance with the invention comprises a measuring device arranged in the paper machine before or at the beginning of the dryer unit to measure the moisture profile of the paper web by infrared measurement prior to drying the paper web, the measuring device being arranged to measure the cross direction profile of the web substantially simultaneously. The measuring device comprises optical fibers each of which is arranged to transmit infrared radiation to the paper web and optical fibers each of which is arranged to transmit the radiation from the paper web at a measuring point, measuring heads in which the ends of the optical fibers are respectively arranged, the measuring heads being spaced apart in a cross-direction of the paper web, and an oscillation means for making the measuring heads move back and forth over part of the web width, perpendicularly to the travel direction of the paper web, such that the measuring heads substantially simultaneously measure web moisture at a plurality of measuring points distributed across substantially a full width of the paper web so as to measure a cross-direction profile of the web moisture across substantially the full width of the web. The apparatus also includes a profiling device for controlling the moisture profile of the paper web on the basis of the moisture profile measured prior to drying the paper web.
The basic idea of the invention is that the moisture profile of the paper web is measured prior to drying and on the basis of said measurement alone the moisture profile of the paper web is controlled prior to drying. Further, it is essential that the measurement is performed with a measuring device which measures the cross direction profile of the web substantially simultaneously. The basic idea of one preferred embodiment is that the moisture profile is also measured after the drying and on the basis of said measurement the moisture profile of the paper web is controlled during the drying.
The invention has an advantage that the moisture profile can be equalized when the dry matter content is low before the paper web moves to a dryer section, and consequently, before the web will have its final structure, it is possible to eliminate defects resulting from the combined effect of a possible moisture profile defect and differences in draw in the drying section. Thus, paper web defects in tensile stiffness module and stretch at break can be eliminated, and a defect in the tensile profile can be prevented. Further, edge defects in the web, edge elongation and web flutter can be reduced. All in all, it is possible to reduce defects produced during the drying of paper, and this results in a more uniform quality of paper, better reelability and better runability also in further processing. By regulating the wet draw it is also possible to control paper properties, such as tensile stiffness module and stretch at break, in a desired direction. The moisture profile having been equalized already before the dryer section, there will be no need to overdry the paper in the dryer section in order to equalize the moisture profile. Hence, the solution increases drying capacity, because average moisture in the paper can be higher than previously. In addition, by measuring the paper moisture substantially simultaneously across the paper web, it is possible to obtain an accurate CD resolution of the measurements, which is of assistance in the web drying. The simultaneous profile measurement also makes the control faster, as compared with the traversing measurement that is currently in common use.
In the present document, the term xe2x80x98paperxe2x80x99 refers not only to paper, but also to paper board and tissue.